The hydrogenated products have wide ranging applications in fine chemicals, pharmaceuticals and pertrochemical industries and this invention relates to a significant improvement in the catalytic process by interfacial catalysis, as illustrated by a variety of substrates with different functional groups.
Hydrogenation reactions are industrially important for the manufacture of a wide range of compounds. These products find applications in fine chemicals as well as intermediates in pharmaceutical and petrochemical industries. Hydrogenation reactions using homogeneous catalysis are well known in which addition of hydrogen to a substrate in the presence of a catalyst soluble in the reaction medium is involved. Although, this route has been applied commercially only in a few cases, it is of potential importance for systems where selectivity as well as activity are of prime importance. The major disadvantage in the use of homogeneous catalysis in hydrogenation is the difficulty in separation and isolation of the product from the catalyst solution and recycle/recovery processes.
To overcome these disdvantages various attempts were made to heterogenize these catalysts by binding the metal complex on supports like silica, polymer etc. These methods have so far not provided a commercially viable heterogeneous catalyst since loss of activity, selectivity, leaching, deactivation and decomposition of the catalyst is observed on repeated use (Kalck and Monteil in Adv. Organomet. chem 34, 219-284, 1992 and Bailey and Langer in Chem. Rev. 81, 109, 1981).
A major breakthrough in this direction has been the synthesis of water soluble phosphine ligands (E. Kuntz U.S. Pat. No. 4,248,802, 1981). These water soluble phosphine ligands are generally synthesized by introducing a hydrophilic group on the ligands. These phosphines generally occur in two major classes (1) Phosphines containing quarternized salt as the hydrophilic component e.g. (i) [2 (diphenyl phosphino) ethyl]trimethyl ammonium salt (amphos) (Smith & Baird in Inorg. Chim. Acta. 62,135, 1982) (ii) [2 (diphenyl phosphino) ethyl]trimethyl phosphonium salt (phosphos) (Renaud et. al. in J. organomet. chem. 419, 403, 1991) (2) Phosphines containing sulphonated groups as the hydrophilic component. Triphenyl phosphine monosulphonate sodium salt (TPPMS) and triphenyl phosphine trisulphonate sodium salt (TPPTS).
Besides these major classes, other modifications of phosphines find limited applications (Kalck and Monteil in Adv. Organomet. chem 34, 219-284, 1992 and reference cited therein). These ligands have been used for-the formation of water soluble complexes of transition metals. Such complexes are used as hydrogenation catalysts in two phase (aqueous/organic) systems. The system consists of an aqueous phase, comprising of the metal complex along with the water soluble ligand. The organic phase consists of a reactant with or without water immscible solvent. The reaction occurs in the aqueous phase with dissolved reactants. The products (usually water insoluble) separate out into the organic phase thus making product separation and catalyst recycle/recovery easy. The application of this methodology is, however, restricted to the substrates having marginal solubility in water. Due to a very low solubility of most of the organic compounds in the aqueous catalytic phase, the rates of reaction using these catalyst are significantly lower than the conventional homogeneously catalysed systems.
Use of a co-solvent has been advocated to overcome such limitations (I. Hablot Chem. Engg. Sci. 47, 2689 1992). This approach, however, does not prove to be very useful because of complications involving (i) reactivity of co-solvent (ii) enhanced solubility of water in the organic phase causing leaching of the catalyst (iii) large volumes of co-solvent required.
European patent EP 362037, 1990 deals with the preparation of saturated aldehydes from a, b-unsaturated aldehydes in a biphasic system using Rh catalyst and a water soluble ligand--TPPTS. The organic phase comprises of toluene alongwith the substrate.
Complexes of group VIIA VIIIA and IB elements with TPPTS and optional additional water soluble ligand were utilised for hydrogenation of different substrates in EP 372313, 1990. A Co.sub.2 (CO).sub.8 /TPPTS catalyst system has been reported for hydrogenation of substrates like cyclohexene etc. in the same patent.
Complexes of group VIIIA elements with TPPTS ligand as hydrogenation catalyst are reported in German Patent DE 3840600, wherein Rh(NO)(TPPTS).sub.3 complex was used to hydrogenate cyclohexene and cyclooctene.
The hydrogenation catalysts described in the prior art using a biphasic system have a drawback of lower rates due to reactants solubility limitations in the aqueous phase.